1. Field of the Invention
The present invention relates to an optical disc signal processing apparatus and a medium having a program recorded thereon for controlling an optical disc apparatus.
2. Description of the Related Art
Currently, optical disc apparatuses that reproduce information from an optical disc (such as CD (Compact Disc), DVD (Digital Versatile Disc)) are in widespread use. An optical disc apparatus emits a laser beam having the wavelength corresponding to the standard of an optical disc (CD: 780 nm to 790 nm, DVD: 650 nm to 660 nm etc.) and applies the laser beam to a track formed spirally in an information recording layer of the optical disc. An error correction code, modulation, etc., are applied to information, and pits respectively having mark lengths (3T to 11T) corresponding to a signal obtained as a result of the application are formed on the track of the optical disc. The optical disc apparatus produces an RF (Radio Frequency) signal by photo-electrically converting reflected light of the laser beam that have been applied to the track of the optical disc. The optical disc apparatus produces a binarized signal by comparing the level between the RF signal and a DC (Direct Current) signal at a predetermined level, in a binarizing circuit.
The production of the binarized signal based on the RF signal will be described hereinafter. When information is accurately recorded on the track of the optical disc, the RF signal has a waveform that is symmetrical about the intermediate level between the maximum level and the minimum level (hereinafter referred to as “symmetrical waveform”) as shown in FIG. 10A. When the RF signal has a symmetrical waveform, at the intermediate level of the RF signal, jitter included in the binarized signal (fluctuation of the binarized signal on a time axis relative to a clock at a predetermined frequency generated to be phase-locked to the binarized signal (hereinafter referred to as “reproduction clock”)) becomes minimum (the bottom jitter). To make the duty ratio between the high level and the low level of the binarized signal to be 50, the binarizing circuit produces a DC signal having the level equal to the intermediate level of the RF signal by feedback-controlling the binarized signal. That is, when the duty ratio of the binarized signal is 50 and the RF signal has a symmetrical waveform, the binarizing circuit produces the DC signal with the level at which the jitter included in the binarized signal is minimized. The optical disc apparatus executes a process to the binarized signal based on the reproduction clock, and applies demodulation, error correction, etc. It becomes possible therefore to execute excellent reproduction of the information, where the influence by the jitter is minimized (see Japanese Patent Application Laid-Open Publication No. 2005-276289).
However, there exists a low quality optical disc where the intermediate level of an RF signal is not the same as the level at which the jitter included in the binarized signal is minimized, due to recording error of information to the optical disc, etc. As an example, there exists an optical disc where the light amount of reflected light obtained from a pit having the mark length of 3T is smaller than the light amount of reflected light obtained when a pit having the mark length of 3T is accurately recorded, etc. In this case, the waveform of the RF signal becomes, for example, a waveform having a deviation in terms of symmetry (asymmetry), as shown in FIG. 10B, and the intermediate level of the RF signal does not become the same as the level at which the jitter included in the binarized signal is minimized. Therefore, in the binarizing circuit, when comparing the level between the RF signal and a DC signal having an intermediate level of an RF signal that is different from the level at which the jitter is minimized, the jitter included in the binarized signal may be increased. Therefore, the influence of the jitter in reproducing information based on this binarized signal is increased so that excellent reproduction of information may be difficult. Especially, for example, when many pits respectively having short mark lengths (such as 3T and 4T) are used to increase the information amount recorded in an optical disc, the above problem, caused by the difference between the intermediate level of the RF signal and the level at which the jitter included in the binarized signal is minimized, may be more serious.